We propose to develop and clinically evaluate new non invasive diagnostic techniques using ultrasound to assess the mechanical strength of bone in health and disease. The availability of such a rapid, sensitive method should be of considerable importance in studying and treating patients with a variety of clinical problems: in particular, osteoporosis; fracture healing; and populations prone to fatigue or stress fractures. Furthermore, such methods might provide an objective measurement of bone accretion induced by electrical stimulation. Extensive in vitro work on the elastic properties of bone has established relationships between elastic modulus, breaking strength, and density. We have inferred from these results that the velocity of sound through bone should be related to its breaking strength, which in turn is dependent on the extent of its fatigue weakening. Our post-mortem studies of equine third metacarpal bones (MC III) have established a direct relationship between the velocity of ultrasound through bone and the degree of mechanical fatigue. Preliminary research on living horses legs has shown that measurement of dilational acoustic propagation through bone provides an accurate comparative technique. A controlled animal experiment will be carried out to establish the relationship between the velocity of sound measured in vivo and the mechanical properties of those bones measured after sacrifice. Surface wave propagation on bone will be explored as a means of following changes on periosteal bone. A human studies program will be undertaken in parallel with the animal experiment. Initially transmission through the epicondyles of the humerus, medial-lateral borders of the patella and subcutaneous border of the ulna will be examined to determine optimum measurement sites. Radiologic density determinations will be made. We will then study using ultrasound the response of bone to stress and rest; response to stress in atheletes, spinal cord injury patients, osteoporosis, and fracture healing.